The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.
In wireless communications systems, for example Global System for Mobile Communications (GSM), a signal transmitted from User Equipment (UE) to a Base Station (BS) may propagate via multiple paths. Each of the paths may be characterized for example by an attenuation and a delay that are introduced in to the transmitted signal.
In order to have an estimate of a radio channel, the UE may transmit to the BS a training sequence that is known to the BS. The radio channel between the UE and the BS may be estimated by analysing distortions caused by the radio channel to the training sequence. The channel estimation quality may be degraded for example when simultaneous transmissions during the transmission time slot of the training sequence. The degradation caused by the simultaneous transmissions occur to the training sequence may be difficult to compensate for. The simultaneously transmitted training sequences may combine constructively or destructively at the BS so that the BS cannot estimate the effects of the radio channel on the training sequence transmitted by a single UE.
Accordingly, two or more UE transmitting simultaneously using the same training sequence in the same time slot and frequency may cause severe degradation in performance of channel estimations.
Simultaneous transmissions employing the same training sequence may be particularly problematic when the transmissions overlap completely. This may happen in a synchronized network, e.g. a GSM network, where two or more UE transmit a burst simultaneously. When a transmission of a training sequence is distorted by an overlapping simultaneous transmission of the same training sequence, the cause of distortion may be difficult to detect at the receiver, e.g. the UE or the BS.
A frequency re-use factor in a communications network may define a distance between BSs that use the same frequency band. With a tight re-use factor, e.g. 1/1, neighbouring BSs may operate on the same frequency band. Consequently, received training sequences in a BS may be significantly distorted by transmissions from a neighbouring BS using the same frequency band. This distortion may be even more significant in a synchronized network, where transmissions of the same training sequence may overlap completely.
In a GSM network simultaneous transmissions of the same training sequence may be prevented by employing different training sequences in BSs. However, the number of available training sequences is limited, for example in GSM there may be only eight different training sequences available. Therefore, the eight training sequences are reused between the BSs in the GSM network.
The development in 3GPP GERAN, aiming at improved utilization of radio resources, has resulted in the development of orthogonal sub-channels under a VAMOS work item. This multiplexing method is defined in 3GPP TS 45.002 V9.2.0 (2009-11) 3rd Generation Partnership Project; Technical Specification Group GSM/EDGE Radio Access Network; Multiplexing and multiple access on the radio path; General description (Release 9). VAMOS allows multiplexing of transmissions of two users simultaneously on the same radio resource in orthogonal sub-channels. Transmissions of two users in the same radio resource in orthogonal sub-channels need a pair of different training sequences. Consequently, in a VAMOS sub-channel pair, the number of training sequences used by one radio channel is doubled. When VAMOS is applied with the existing training sequences to non-VAMOS handsets, the re-use of eight possible training sequences is more frequent. This reduces the distance between neighbouring base stations that employ the same training sequences. Consequently, the possibility of simultaneous transmissions of the same training sequence in the same time slot and frequency increases.
The number of reference signals may be limited also in other radio network technologies. In Long Term Evolution (LTE) developed by 3rd Generation Partnership Project (3GPP), enhanced Node-Bs (eNBs) may operate using the same frequency with limited number of reference signals. The number of so called Zadoff-Chu root sequences in the LTE uplink may be limited to 30 sequence groups, which can be circularly extended in a frequency domain to obtain a larger number of sequences. The use of Multi-User Multiple-Input and Multiple-Output (MIMO) technology will also multiply the need for reference signals at the same time in the same eNB. The number of pilot signals may be mostly constrained, when the narrowest bandwidth option (1.4 MHz) is linked with a high number of antenna ports. Consequently, the distance between neighbouring eNBs employing the same reference signal may be small also in LTE, and reference signals may cause interference to neighbouring eNBs.
Requirements set for training sequences used in communications systems may include for example Constant Amplitude and Zero Autocorrelation (CAZAC), which reduces the number of possible training sequences that can be used in communications systems. Accordingly, increasing the number of available training sequences in the networks may be difficult.